Liquid fuel as a propellant for rockets has been used to provide the required thrust for many years. Liquid propellants have an advantage over solid propellants because the engine can be started and stopped by controlling the fuel flow to the combustor. In the liquid-fueled rocket engine, the fuel is burned in a fuel rich environment with an oxidant, usually liquid oxygen (LOX), to provide thrust generated by high speed ejection of exhaust gases. In a rocket application, the fuel may be subjected to high environmental temperatures such as when the fuel is used as a coolant prior to combustion, and may decompose resulting in unwanted deposits, gums, foulants or the like. Therefore, the fuels used as rocket propellants should exhibit good thermal stability.
An improvement in thermal stability as compared with more conventional fuels (e.g., RP-1) is particularly important for fuels that are intended for use with reusable rocket engines and in air breathing hypersonic vehicles that employ the fuel as a coolant to help reduce the high temperatures of airframe structures and engine components that are developed during hypersonic flight. The term “air breathing” refers to a vehicle having an engine that is configured to receive air from the atmosphere to be used in combustion of a fuel, and encompasses vehicles having various jet engines, such as ramjets, scramjets, etc.
The specific impulse (Isp) is a performance measure for rocket propellants that is equal to units of thrust produced during ejection of exhaust gases from a rocket engine per unit weight of propellant consumed per unit of time, and therefore specific impulse is measured in units of time (e.g., seconds). Isp can be used to determine the payload that a rocket can carry into orbit. Propellants with a higher specific impulse are desirable in order to deliver a payload into a desired orbit at a minimum cost. It is also desirable that the fuel burns or combusts cleanly and does not form deposits when a portion of the fuel is used for driving a turbine to operate a pump to deliver the fuel to the rocket engine.
Rocket scientists have determined that the specific impulse can be calculated from the equation:
      I    sp    =      9.80    ⁢                            T          c                M              ⁢                  k                  k          -          1                      ⁢                  1        -                              (                                          P                e                                            P                o                                      )                                              k              -              1                        k                                              Where,        M=a weighted average of the molecular weights of the combustion products        Tc is the combustion chamber temperature in degrees Rankine        k=Cp/Cv is the ratio of specific heats of the combustion products        Pe/Po=ratio of external pressure to combustion chamber pressure        
Therefore, to achieve highest Isp it is desirable to have a high combustion temperature (high net heat of combustion) and have combustion products with a lowest possible molecular weight. For example, maximum Isp for any liquid propellant is provided by liquid hydrogen fuel, with oxygen as oxidant, because the product of combustion is only water (M=18). In contrast, a hydrocarbon fuel results in combustion products comprising CO2 (M=44), CO (M=28) and water. Therefore, to maximize Isp, the hydrocarbon fuel must have a high hydrogen content (i.e. a high H/C atomic ratio) and it must burn such that CO2 formation and unburned hydrocarbons are minimized. To minimize CO2 generation and maximize carbon monoxide generation, the rocket engine is designed to combust the fuel under fuel rich conditions.
The disclosed formulations are capable of producing a higher Isp than that provided by a conventional petroleum based refined kerosene called RP-1. The RP-1 specifications were developed for military purposes as MIL-P25576 in 1957 and set a broad criterion for propellant properties with higher density, cleaner burning, ease of handling and performance relative to kerosene jet fuel. Commercially available RP-1 fuels are limited to a hydrogen content of about 14 wt. %, a hydrogen to carbon atomic ratio (H/C) less than 2.0, a heat of combustion less than 18.7 KBtu/lb and can have up to 5 percent by volume aromatics and 2 percent by volume olefins. This conventional RP-1 fuel can also contain up to 30 ppm (weight basis) sulfur. The aromatics and olefins can cause deposits and coke formation in the cooling chambers and sulfur can cause rapid corrosion.
Previously, cycloalkanes, such as 1,2-diethylcyclohexane (DECH), which have optimum density and combustion properties, were added to refined kerosene to produce a rocket propellant. However, DECH has a molecular formula of C10H20 with a density of about 0.80 g/cm3 while only having an H/C atomic ratio of 2.0. Therefore, addition of DECH to conventional RP-1 does not provide a great improvement. Further, DECH is not readily available in large quantities. Substituted cycloalkanes are also believed to produce combustion chamber products with a higher molecular weight than the breakdown products from isoparaffins.